In manufacturing of semiconductor devices, a cleaning process with the use of a chemical liquid is performed in order to remove unnecessary films or contaminants on a substrate such as a semiconductor wafer. A substrate processing apparatus for performing such a cleaning process is provided with a process liquid circulation system for supplying a process liquid to liquid processing units incorporated in the substrate processing apparatus (See JP2007-329328A, for example.).
The process liquid circulation system provided in the substrate processing apparatus of JP2007-329328A includes a process liquid storage tank and a circulation line (pipeline) connected to the process liquid storage tank. The circulation line has a pump, by which a process liquid is fed out from the process liquid storage tank and is returned to the process liquid storage tank. Liquid processing units are connected in parallel to the circulation line through respective branch lines. In accordance with process schedules of the respective liquid processing units, the process liquid is supplied to each liquid processing unit through each branch line, and each liquid processing unit performs a predetermined liquid process to a substrate by using the process liquid. When the process liquid in the process liquid supply system deteriorates, the deteriorated process liquid is changed or replaced with a new process liquid of the same kind. When a different process using a process liquid of different kind or different concentration is performed in the substrate processing apparatus, the process liquid in the process liquid supply system is changed or replaced with the process liquid of the different kind or the different concentration.
A conventional process liquid changing method is carried out by (1) discharging an old process liquid from a process liquid storage tank, (2) feeding a new process liquid to the process liquid storage tank, (3) causing the new process liquid in the process liquid storage tank to circulate in a process liquid circulation line for a predetermined period of time, so as to expel the old process liquid in the process liquid circulation line therefrom, (4) discharging the process liquid from the process liquid storage tank, and (5) repeating steps (2) to (4) plural times. Since it is difficult to completely remove the liquid existing in the process liquid circulation system (in particular, in a piping), a certain amount of liquid inevitably remains in the process liquid circulation system. If the amount of liquid remaining in the process liquid circulation system is large, the aforementioned steps (2) to (4) should be performed many times, resulting in wasteful consumption of the process liquid.